From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156707-523771
Question: I recently dug a border garden which contained a lot of
crabgrass and other weeds. I would like to begin planting some
perennials/small trees and ground cover. What kind of weed killer can
I use to prevent the weeds from coming back, and should it be applied
prior to or after planting the new flowers?
Answer: There are some pre-emergent herbicides on the market that can
be incorporated into the top 3" of soil after planting your
perennials. Or, you can line the bed with one of the poly-spun fabric
weed barriers, cutting holes in the fabric to plant your perennials,
shrubs and trees. You can cover the weed barrier with decorative rock
or bark, or plant groundcover to disguise its presence.